This invention concerns a sigma delta modulator for use in a voice codec application used to convert analog voice signals to digital signals such as those used with GSM.
In quantizing operation in A/D conversion, partial missing of information in the quantized output signal is inevitable because continuously distributed sampled values in an input signal are expressed by dispersed values in the quantized output signal. This difference between the quantized output signals and the input sampled values becomes a quantizing noise which is an inherent restriction on the digital signal processing. The quantizing noise becomes a white noise which is uncorrelated with a signal to be converted if the signal is of a large level whereas the quantizing noise becomes distortion which is greatly correlated with the signal if the signal is of a relatively small level and this noise adversely affects the quality of tone when a signal is sounded in digital apparati.
A dither circuit can be used to overcome this problem. The dither circuit is provided for uncorrelating quantizing noise with a signal to be converted by superimposing dither or random noise on an input signal before A/D conversion and subtracting the dither component after the A/D conversion.
Input range adjustment is strongly desired under low power supply operation since the signal swings must be limited to the range power supply and ground rail. Some of the problems inherent in the prior art include asymmetrical signal clipping and shifting in sigma delta modulator circuits.